The invention concerns a process for the separation of oxygenated compounds (methanol, dimethyl ether, water) from a mixture of hydrocarbons containing 3 to 8 carbon atoms, and the use thereof.
Alkyl tertioalkyl ethers are used as high octane number additives for lead-free or reduced lead petrols (i.e., fuels).
The most frequently used alkyl; and tertioalkyl ethers are produced by addition of methanol to a tertiary olefin.
Methyl tertiobutyl ether (MTBE), for example, is obtained by the addition of a molecule of methanol to a molecule of isobutene.
Similarly, tertioamylmethyl ether (TAME) is obtained by the addition of a molecule of methanol to a molecule of isoamylene.
These ethers are normally synthesized in a process generally involving introducing a liquid phase comprising a certain quantity of methanol and the tertiary olefin(s) required for the reaction into one or more reactor(s) containing a suitable catalyst. A high conversion factor for the tertiary olefins requires an excess of alcohol to be introduced which must then be separated from the effluent from the reaction zone for recycling. The tertiary olefins are generally present in a hydrocarbon mixture. Isobutene used for MTBE synthesis, for example, is most often contained in a C4 steam cracking or catalytic cracking cut. The hydrocarbons which are thus introduced into the reaction zone with the tertiary olefin(s) do not usually react, or react only slightly, and are thus also found in the effluent from the reaction zone.
The effluent from the reaction zone is thus a mixture containing the ether produced by the reaction, excess unreacted alcohol, unreacted hydrocarbons and small quantities of products produced in secondary reactions such as methanol etherification which produces a molecule of water and a molecule of dimethyl ether from two molecules of methanol.
A variety of processes for the separation of this mixture have been proposed. The most frequently employed process consists in sending the effluent from the reaction zone to a distillation column to produce an effluent containing all the ether at the bottom along with, possibly, a portion of the alcohol present in the effluent from the reaction zone. A raffinate constituted by the hydrocarbons and a portion, preferably all, of the alcohol present in the effluent from the reaction zone, is taken overhead. The alcohol content in the raffinate is generally 0.1% to 10% by weight, preferably 1% to 5% by weight. The alcohol must then be separated from the raffinate for recycling to the reaction zone. The raffinate also contains water and dimethyl ether, produced in secondary reactions, as impurities. Even though these oxygenated products are present in small quantities (generally 0.01% to 0.1% by weight), in general they too must be eliminated from the raffinate. Once purified, the raffinate is usually used as a feedstock for other reactions (for example, alkylation); because of the catalysts used in these subsequent reactions, severe restrictions are usually imposed on the total amount of oxygenated compounds (1 to 50 ppm depending on the case).
In certain processes, the effluent from the reaction zone is sent to a catalytic distillation column, either to increase the tertiary olefin(s) conversion rate or to reduce the excess of alcohol used. Whatever the case, the raffinates taken overhead from the catalytic distillation columns have analogous compositions to those of raffinates taken overhead from conventional distillation columns, but with slightly lower alcohol contents.
The methanol contained in the raffinate cannot be eliminated by simple distillation because of azeotrope formation between the methanol and the hydrocarbons. It can, however, be removed by various methods. The most popular process used is described in U.S. Pat. No. 3,726,942 and involves washing the raffinate with water. This process is efficient but requires distillation of the water/alcohol mixture thus produced in order to recycle the alcohol to the reaction zone. This distillation step makes the water washing process fairly expensive as regards investment and energy consumption.
A further water washing process is described in European patent EP-A-459,627.
Other variations of this water washing process have been described. U.S. Pat. No. 4,118,425, for example, describes a process in which the effluent from the reaction zone itself is washed with water before entering the distillation column, a raffinate which is free of alcohol being taken overhead. Here too, the water/alcohol mixture produced during the washing step must be distilled to recycle the alcohol to the reaction zone.
The water washing process has the further disadvantage of not being able to eliminate oxygenated impurities such as dimethyl ether from the raffinate, whichever variation is used. Still further, the water washed raffinate is, of course, saturated with water.
U.S. Pat. No. 4,740,631 describes a process in which the methanol is eliminated from the raffinate by adsorption on an appropriate molecular sieve (zeolites) with subsequent regeneration of the molecular sieve by the hydrocarbon mixture constituting the feedstock for the reaction zone. This process has the advantage of removing the distillation step required by water washing processes. Unfortunately, as with all adsorption processes, this is a discontinuous process since the molecular sieve used has to be regenerated periodically. This means that at least two adsorption beds and a relatively complex operating procedure must be used to carry out the alternating cycles of adsorption and regeneration. In addition, as with water washing processes the adsorption process cannot eliminate the dimethyl ether present in the raffinate. Here again, subsequent purification steps are necessary, for example by adsorption on other molecular sieves or by distillation, in order to achieve the low total oxygenated compound contents generally required.
A simpler methanol elimination process is described in U.S. Pat. No. 4,740,632. In this process, the raffinate is constituted by a mixture of methanol and hydrocarbons containing 4 carbon atoms. Since the hydrocarbons are then sent to an alkylation unit which is catalysed by sulphuric acid, the spent sulphuric acid from the alkylation unit is used to treat the raffinate. This produces an organic phase containing methanol-free hydrocarbons which is sent to the alkylation unit and an aqueous phase containing the methanol which has reacted with the sulphuric acid. This aqueous phase is then sent to a regeneration unit to recover the acid. While this process is very simple, it is quite expensive when the raffinate to be treated contains more than 0.5% by weight of methanol, which is often the case, since the methanol which has reacted with the acid cannot be recovered during the acid regeneration step for recycling to the reaction zone.
Another method of separating the alcohol in the raffinate is to bring the mixture for separation into contact with a membrane which is selectively permeable either to the alcohol, which is the most frequent and most advantageous case bearing in mind the small amounts of alcohol to be extracted, or to the hydrocarbons.
U.S. Pat. No. 4,759,850 describes a method of separating methanol from a mixture of hydrocarbons and/or ethers by reverse osmosis.
Better selectivities, and thus purer products, have however been obtained by pervaporation. In this process, the mixture for separation is brought into contact, as a liquid and at an appropriate temperature and pressure, with one face of a membrane. A vacuum is applied to the other face. The membrane is selective to one of the constituents of the mixture which preferentially diffuses through the membrane. A permeate is recovered downstream of the membrane (vacuum side) which is enriched in this constituent and which can then be compressed or condensed at low temperature. From the upstream side of the membrane, a residue is obtained containing of the initial mixture depleted in the constituent. Because of its selectivity, this process is of particular advantage when azeotropic mixtures are to be separated.
Most of the pervaporation membranes in current use are selective to water in mixtures of organic products. There are some, however, which are selectively permeable to alcohols in organic mixtures.
U.S. Pat. Nos. 4,798,674, 4,877,529, 4,960,519, 5,152,898, German patent DE-A-4,234,521 and European patent application EP-A-92117467.8 describe various such membranes and their use in extracting alcohols with less than three carbon atoms, preferably methanol, by pervaporation from mixtures containing other oxygenated organic compounds such as ethers, esters, aldehydes or ketones.
U.S. Pat. No. 4,774,365 describes a process for the separation of alcohol present in the effluent from the reaction zone in an etherification process using a pervaporation membrane.
In a first embodiment of the process, the effluent from the reaction zone passes over a pervaporation membrane which selectively extracts alcohol. The extracted alcohol is recycled to the reaction zone. The alcohol-depleted residue obtained is then distilled. In this embodiment, an alcohol-free ether can be obtained from the bottom of the distillation column. However, in order for the raffinate taken overhead from this distillation column to be completely free of alcohol, all the alcohol present in the effluent from the reaction zone must have been eliminated by the pervaporation unit. It is, however, known that pervaporation, in common with all processes using membranes, becomes very expensive when the last traces of a product have to be removed (in general, dropping to concentrations of less than 0.1% is not economical). Thus it is difficult to use this embodiment to produce a raffinate with a very low alcohol content.
The same document describes another embodiment in which the effluent from the reaction zone is sent directly to the distillation column. A liquid fraction is removed as a side stream from the column and sent to a pervaporation membrane which extracts a portion of the alcohol present in this liquid fraction. The extracted alcohol is recycled to the reaction zone while the alcohol-depleted residue is returned to the distillation column. Again, this embodiment produces an alcohol-free ether at the bottom of the distillation column. However, because alcohol is entrained in the hydrocarbons by azeotropy, it is still difficult for a raffinate (the value given in Example 2 of the patent document) containing less than 0.1% by weight of residual alcohol to be taken overhead from the distillation column. This remains true for all combinations of the two embodiments, as shown in Example 2 of the document.
Further, apart from using a membrane which will simultaneously extract methanol and dimethyl ether from a mixture containing hydrocarbons and/or other ethers, it appears clear that, whatever the embodiment used, this process cannot eliminate or reduce the amount of dimethyl ether present in the effluent from the reaction zone which then appears in the raffinate.